Combination antiretroviral therapy can control HIV-1 replication and delay disease progression. However, despite the complete suppression of detectable viremia in many patients, viremia reemerges rapidly after interruption of treatment, consistent with the existence of a latent viral reservoir. This reservoir is thought to consist mainly of latently infected resting memory CD4+ T cells. Due to the long half-life of this reservoir (44 months), it has been estimated that its total eradication with current treatment would require over 60 years.
Latently infected cells contain replication-competent integrated HIV-1 genomes that are blocked at the transcriptional level, resulting in the absence of viral protein expression. HIV depends on both cellular and viral factors for efficient transcription of its genome, and the activity of the HIV promoter is tightly linked to the level of activation of its host cell. It is thought that reactivation of latently infected memory T cells by their cognate antigen leads to a reactivation of viral gene expression and the completion of the viral life cycle. However, it is not clear how the latent state is established. It has been proposed that latency occurs when an activated T cell in the early stage of infection returns to a quiescent state, leading to suppression of viral transcription until the cell becomes reactivated. However, the infected cell would need to survive the cytopathic effects of infection to effectively transition to the resting state. Alternatively, an activated, infected cell could become quiescent before the onset of viral expression and the occurrence of cytopathic effects, as has been reported during thymopoiesis. The unlikely coincidence of these two events could account for the low frequency of latently infected cells in vivo (˜106 cells per infected individual).
HIV transcription is characterized by two temporally distinct phases. The early phase occurs immediately after integration and relies solely on cellular transcription factors. Because of a transcriptional elongation defect in the basal HIV promoter, most transcripts cannot elongate efficiently and terminate rapidly after initiation. This process leads to the accumulation of short transcripts at the 5′ region of the viral genome containing the TAR element. However, the elongation defect is not absolute, and a few transcripts elongate throughout the genome, resulting in transcription of the viral transactivator Tat. The late phase of transcription occurs when enough Tat protein has accumulated. Tat binds to TAR, recruits the pTEFb complex, and causes the hyperphosphorylation of RNA polymerase II, dramatically increasing its ability to elongate.
To understand how postintegration latency is established and to test novel therapeutic approaches for the reactivation of these viral reservoirs, an in vitro cell system reflecting the state of HIV-1 latency is required. Several HIV latently infected cell lines harbor defective proviruses raising significant questions about their significance in understanding the mechanism of latency in vivo. The latent cell lines ACH2 (T cell) and U1 (promonocytic) contain HIV proviruses that harbor mutations in their Tat-TAR transcriptional axis. Another chronically infected cell line, J-delta-k (T-cell), harbors an HIV-1 provirus lacking NF-κB binding sites in the HIV promoter. These observations suggest that inhibition of transcription is critical to establishment and maintenance of latency.
There is a need in the art for an in vitro cell system that accurately reflects latent immunodeficiency virus infection in vivo. The present invention addresses this need.
Literature
Kulkosky et al. (2001) Blood 98:3006-3015; Emiliani et al. (1996) Proc. Natl. Acad. Sci. USA 93:6377-6381; Emiliani et al. (1998) J. Virol. 72:1666-1670; Antoni et al. (1994) Virol. 202:684-694; Carteau et al. (1998) J. Virol. 72:4005-4014; U.S. Pat. Nos. 6,225,048; 6,025,124; 5,459,056; and 5,256,534. See U.S. Pat. No. 6,025,124 for a discussion of U1 cells; see U.S. Pat. No. 5,459,056 for a discussion of ACH-2 cells. See U.S. Pat. No. 5,256,534 for a discussion of OM-10.1 cells.